textproduct: Rapid City

KEY MESSAGES

- Continued warm and breezy conditions today and Friday with a very modest increase day-by-day in the potential for a couple isolated showers or thunderstorms; most areas to remain dry.

- Slight Risk of severe thunderstorms Saturday afternoon and evening with much more widespread development and primary hazards of large hail quarter to golf ball size, damaging winds in excess of 60 MPH, and heavy rainfall.

- Unsettled conditions with daily chances of showers and thunderstorms continue through at least Tuesday next week.

UPDATE

Issued at 1056 PM MDT Thu May 28 2026

There is some uncertainty in the forecast highs for tomorrow. Temperatures tomorrow will be influenced by where cloud cover and precip occur. The coolest conditions will be across the eastern third of our CWA where showers and cloud cover should keep temperatures on the cooler side. NBM guidance is running "hot" with highs in the mid to upper 80s across the western half of our CWA while MOS and hi res models are coming in 5-10 degrees cooler. Have knocked down tomorrow's highs slightly to account for this.

As for Saturday, the forecast remains on track for widespread shower/storm activity and the risk for severe storms across western SD. 00z CAMs are still showing initial storm activity developing across northeastern WY early Saturday afternoon with stronger storms moving into western SD later in the afternoon.

DISCUSSION

(This Evening Through Thursday) Issued at 232 PM MDT Thu May 28 2026

That warm and breezy High Plains life continues! But, some changes are on our doorstep. Today into Friday, a shortwave-style "tongue" of low pressure will be lifting northward across Kansas and Nebraska with very gradually increasing moisture and instability over our region, supporting a modest increase day-by-day in isolated shower and thunderstorm potential. Cloud cover will be a limiting factor to our destabilization (especially today but perhaps Friday as well) with model forecast soundings generally supporting less than 1,000 J/kg MLCAPE. Lift will be lacking too and continued very poor shear parameters will do nothing to help us best utilize that CAPE or support/sustain any storm organization. The result will be weaker "pulse" activity with limited coverage, mostly focused on Friday afternoon/evening with some cells affecting northeast Wyoming and perhaps firing off the Black Hills generally after 2-3pm. Most of our forecast area is still likely to remain dry both days (although some guidance tries to generate some showers late Saturday night into Sunday morning, especially across our South Dakota plains). Generally held with NBM guidance for our forecast, but shaved a few degrees off high temps this afternoon given widespread cloud cover and trends in sfc obs, and continued our theme from previous days in slightly reducing winds/wind gusts as the NBM continues to run hot. Still, widespread gusts of 30-40 MPH will continue both afternoons, with a localized gust to 50 MPH not out of the question in normally breezier spots, especially in the Gillette region and across northwest South Dakota. For our fire partners...minimum afternoon RH values are slowly trending higher compared to previous days despite the breezy conditions...25-35 percent today, and 25-40 percent Friday (except lower across northeast Wyoming). More good news on the moisture front follows below.

By Saturday, synoptic-scale reinforcements arrive as a large, nearly stationary low pressure center that has been residing over California and Nevada lately finally ejects northeast across the Rockies, with the overall trough taking on a strongly negative tilt and cooling mid-level temperatures. Ahead of this feature, strong moisture advection will be underway on south/southeast flow throughout the atmospheric column with sfc dew points reaching 55-65 degF and increasing instability. Surface and near-surface winds backed to southeasterly will assist in transporting this moisture westward at least up against the Black Hills corridor and enhance storm-relative inflow. A review of NAMNest and GFS model forecast soundings...even using MLCAPE or effective parcel CAPE...suggest 1,000 to 2,000 J/kg (moderate instability/fuel for storms), along with PWATs increasing over 1 inch. Bottom line...moisture and instability are looking MUCH more supportive of thunderstorms by Saturday afternoon regionwide, and the NWS Storm Prediction Center (SPC) has introduced a Slight Risk (Level 2 of 5) of severe thunderstorms across most of our South Dakota counties (where these two parameters will be highest) with a Marginal Risk (Level 1 of 5) for the rest of our CWA. (For what it's worth, most analog and machine-learning severe weather probabilities out there support this as well.) Shear parameters will improve too, but will still remain somewhat of a limiting factor to a more robust severe weather threat as deep-layer shear does not currently look to exceed 30kts, resulting in messy multi-cell to low-end supercell storm modes albeit with some decent cell longevity. Those backed surface winds into the SE (especially across our South Dakota counties) as well as some increase in 0-1 and 0-3 km SRH values toward evening indicate the risk of an isolated, brief tornado is low, but not zero. Decent CAPE in the hail growth zone, and initially steep low-level lapse rates coupled with DCAPE solidly over 1,000 J/kg and the higher PWATs suggest primary risks of heavy rain, perhaps some wet microbursts (damaging winds over 60 MPH), and large hail of quarter to golf ball size with model soundings pulling some severe hail analogs. Simulated reflectivity off the NAMNest suggests the stronger storms could initiate as early as 18z/noon in northeast Wyoming, and 21z/3pm into western South Dakota, likely continuing into the first part of the night Saturday night trending into our eastern Plains with time...storms may grow upscale into more of a line or cluster during their journey northeast giving everyone some rain. High temperatures Saturday will top out in the upper 70s to low 80s given some cloud cover and storms running around. NBM guidance captured most elements reasonably well for now and no significant forecast edits were made, although current PoPs prior to sunset Saturday may be a bit overdone. Some locales in South Dakota could see QPF exceeding 0.50 inches between Saturday afternoon and Sunday morning! (For our fire partners, chance of wetting rain will reach 40-80 percent by this period.)

The main low looks to slowly drift northeast Sunday through Tuesday with multiple subtle shortwaves/disturbances rotating around it, resulting in additional daily chances of mostly afternoon and evening showers and thunderstorms across the region. It's possible we start to trend drier after that, but forecast confidence is currently low. Latest 500mb height ensemble cluster analysis still universally supports continued presence of some sort of low/troughing with it's center/axis still west/northwest of our CWA through at least Tuesday, with that signal weakening by Wednesday. Broadly speaking, no significant cool-down is expected at this time as a result of this system and pattern through next Wednesday.

AVIATION

(For the 12Z TAFS Through 12Z Saturday Morning) Issued At 541 AM MDT Fri May 29 2026

VFR conditions are expected through the forecast period. Gusty southeasterly winds (upwards of 25 to 35 kts) will continue today, decreasing over this evening. Isolated showers and storms will be possible this afternoon into tonight, but low confidence in them going over the TAF sites.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.